Ceramics, such as boron carbide, are a hard materials which are also lightweight, extremely stiff and more economical to use than diamonds and cubic boron nitrite, the only other materials known to be harder than boron carbide. Because of low weight and hardness, ceramics are particularly attractive for use as protective armor. However, due to their brittleness, ceramics are relatively easily fractured when stressed. To alleviate this problem, metals such as aluminum have been combined with ceramics to provide ceramic/metal composites that are generally stronger than steel by weight, somewhat less dense than the metal, and several times more resistant to fracture than the ceramic. Other proposed uses for such ceramic/metal composites include low inertia, dimensionally stable, and low vibration rotating components for computer hard disk drives; long-life bearings and races; premium performance sporting goods; high-efficiency, electron emission devices; cutting tools for hard to machine silicon aluminum alloys; and nuclear shielding.
Methods of making such ceramic/metal composite, also known as "cermets," are known. For instance, U.S. Pat. No. 4,605,440 and 4,702,770 disclose boron carbide and aluminum composites with tailorable microstructures. These patents describe forming the composites by infiltrating a monolith of boron carbide with aluminum.
More recently, U.S. Pat. No. 4,718,941 describes boron carbide-, boron-, boride-, and reactive metal composites that are made by infiltrating aluminum into a boron carbide sponge having gradient microstructures. The gradient boron carbide microstructures are described as being provided by controlled colloidal consolidation of boron carbide starting constituents or other techniques such as cold pressing, warm pressing, plasma jet coating, combustion synthesis, hot pressing, hot isostatic pressing, etc.
Despite the existence of these composites, there is still a need for ceramic/metal composites having improved fracture strength and toughness, and for fabrication methods which produce composites having such improved physical characteristics.